delivery-log-storage: the table for a message's log lines

Migration 0003_send_log_lines.sql, DDL only: queue id, the line's own time,
the line. No reference to send_log, because a message's first lines are
logged before the journal milter writes its row and one queue id is several
rows.

Store methods: InsertSendLogLine; InsertSendLogLineOnce for lines the tailer
reads twice after an unclean stop; SendLogLines, which gives a row the lines
of its queue id dated from an hour before the row to the queue lifetime after
it, so a later message under a reused id is not shown; PruneSendLogLines,
which deletes lines whose queue id has no send-log row - the whole of the
table's retention. A line is cut to 4096 bytes.

Nothing writes or reads the table yet. Checked on Windows: go vet and
go test ./internal/store pass, with the schema test pinning the new table,
its index and user_version 3.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,15 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+### Added
+
+- **Schema: `send_log_lines`** (migration `0003_send_log_lines.sql`, DDL only).
+  The table that will hold what Postfix wrote about a message for as long as
+  its send-log row lives, with the store methods to write a line, to write it
+  once, to read the lines of a row and to prune
+  ([plans/delivery-log-storage.md](docs/plans/delivery-log-storage.md)). Nothing
+  writes or reads it yet.
+
 ## [2.1.0] - 2026-10-12
 
 ### Security